Broadband driver with extended linear output voltage

ABSTRACT

Modern modulator drivers must be capable of delivering a large output voltage into a tens of ohms modulator, while minimizing the amount of distortion added by the driver. The driver should deliver the output voltage without exceeding a maximum distortion while minimizing the DC power consumption. Accordingly, a modulator driver includes a final stage amplifier with auxiliary transistors that turn on when the conventional differential pair of transistors approaches their maximum voltage of the linear region of their transfer function, thereby providing a more linear transfer function, in particular at large input voltages.

TECHNICAL FIELD

The present invention relates to a driver for an optical transmitter, and in particular to a broadband driver with extended linear output voltage.

BACKGROUND

Information is transmitted in an optical channel using optical modulation. In a transmitter, the information in the form of an electrical signal is used to modulate an optical signal, which may be modulated in amplitude, phase, polarization or a combination thereof. The modulation of the optical signal is done using a transducer that converts the electrical signal to the modulated optical signal, e.g. Mach-Zehnder modulator. The transducer uses both an electrical and an optical input signal, by modifying the optical input according to the electrical signal, such the optical output signal contains the information to be transmitted. Typically, the electrical input signal is a voltage.

Information is processed locally in the electrical domain, and then for optical transmission the electrical signal is mapped using a modulation scheme, e.g. 64QAM. The resulting electrical signal is amplified using a voltage-to-voltage amplifier called a driver, since its output “drives” the electro-optical modulator e.g. Mach-Zehnder modulator. The optical modulator usually requires a voltage with low distortion and several volts of magnitude.

To increase optical channel capacity complex modulation schemes may be used, e.g. quadrature modulation. Therefore, the amplitude and phase of the electrical signal have constraints that translate to large voltage amplitude, e.g. several volts, and high linearity, e.g. low distortion. A driver circuit generates the electrical signal used by the modulator. Large output voltages, high linearity, and low power consumption are the constraints that make the driver one of the most challenging components to design in an optical transmitter

Typically, the driver input signal has been processed, e.g. by the manipulation of magnitude and phase, to maximize the channel capacity. In order to reduce power consumption, all processing may be done with low power circuits, which limit the maximum voltage magnitude that can be provided to the driver. Accordingly, the function of the driver is to amplify the incoming signal to values required by the modulator while adding minimum distortion.

The driver circuit typically includes several blocks, for example: an input buffer, one or more variable gain amplifiers (VGA), and an output or driver stage. The driver's last stage must deliver the output voltage. In a current-mode logic design, the driver output voltage equals the last stage current times the modulator's impedance. The modulator's impedance may be a value in the order of tens of ohms; however, if several volts of output voltage is required by the modulator, the last stage current must be in the order of tens of milliamps.

The requirements for the output voltage necessitates that the transistors in the driver conduct current ranging from the full current in the final stage to almost no current, i.e. the transistors steer the last stage current in the differential output. When the transistors operate under these constrains, they operate almost as switches turning ON and OFF. Therefore, they are operating in their most non-linear operation mode, and they contribute to the generation of undesired distortion.

An alternative is to trade-off power consumption for linearity. Increasing the current handled by the last stage enables the output transistors to operate in a linear region, for example using a differential pair with increased bias current and increased degeneration. However, this alternative increases the power consumption of the driver.

An object of the present invention is to overcome the shortcomings of the prior art by extending the linear output voltage range of a driver, and reducing the power consumption compared to other solutions with comparable output voltage and linearity.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a transmitter driver for preparing input electrical signals for output to a modular comprising:

first and second inputs for inputting first and second input electrical signals;

a buffer stage for shifting a reference of the first and second input electrical signals;

an amplifier stage receiving the first and second input electrical signals, including:

first and second differential pair transistors forming a differential pair connected to the buffer stage;

first and second outputs connected to second terminals of the first and second differential pair transistors respectively for outputting first and second output electrical signals; and

first and second auxiliary transistors connected to the second terminals of the first and second differential pair transistors,

wherein the first and second auxiliary transistors are capable of turning on when the first and second input electrical signals exceed a maximum input voltage of a linear region of a transfer function of the first and second differential transistors for increasing the first and second output electrical signals, thereby increasing a linear region of a transfer function of the amplifier stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:

FIG. 1 is an schematic diagram of a transmitter in accordance with an embodiment of the present invention;

FIG. 2 is a schematic diagram of a conventional final stage driver;

FIG. 3 is a graph of a normalized transfer function of the driver of FIG. 2;

FIG. 4 is a schematic diagram of a final stage driver with extended linear output voltage of FIG. 1;

FIG. 5 is a graph of a normalized transfer function of the driver of FIG. 4;

FIG. 6 is graph of THD vs normalized output voltage comparing conventional drivers with the driver of FIG. 4;

FIG. 7 is a graph of normalized voltage gain vs normalized frequency comparing conventional drivers with the driver of FIG. 4;

FIG. 8 is a schematic diagram of a final stage driver with cascode output stage and extended linear output voltage of FIG. 1; and

FIG. 9 is a graph of THD vs normalized frequency comparing conventional drivers with the driver of FIG. 4.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.

A driver circuit 1, in accordance with the present invention comprises an input buffer 2, one or more variable gain amplifiers (VGA) 3 _(i) to 3 _(n), and a last stage amplifier 4. The last stage amplifier 4 is responsible to deliver the current required to generate a desired output voltage. A gain controller 5 may be included in the driver circuit 1 or external thereto for sending gain control signals to one or each of the VGA's 3 _(i) to 3 n. The gain controller 5 may receive a gain control signal VG_CTRL from an external source and/or the gain controller 5 may be part of a feedback loop, which compares the levels of the electrical signals from a tap (shown in broken lines) to a desired level and controls the gain of the VGA's 3 _(i) to 3 _(n) accordingly. The driver circuit 1 may be embedded between a digital to analog converter (DAC) 6 for generating an analog signal, which has been digitally processed, and an electro-optical transducer 7, e.g. a Mach-Zehnder modulator. A transmitter may comprise the driver circuit 1 in combination with the electro-optical transducer 7, and a light source, e.g. a laser, 10. Accordingly, the light source 10 generates an optical signal, which may then be modulated using the first and second output electrical signals from the driver circuit 1.

If properly designed, the distortion introduced by the driver circuit 1 may be mainly generated in the last stage amplifier 4. Design and optimization of the last stage amplifier 4 is key to obtain good linearity. The proposed solution description is based in a SiGe Bipolar Transistor technology, i.e. first base terminal, second collector terminal, and third emitter terminal; however, the principle presented may be applied to CMOS or other technologies, e.g. first gate terminal, second drain terminal, and third source terminal.

With reference to FIG. 2, a conventional driver circuit with differential output voltage may comprise a differential pair of transistors Q₃ and Q₄ as the last stage amplifier 4′. In the last stage amplifier 4′, the linear range of the differential pair Q₃ and Q₄ may be limited by their hyperbolic transfer function (See FIG. 3). FIG. 3 illustrates the differential pair transfer function and an ideal linear transfer function. Therefore, nonlinearities increase once the differential pair input voltage exceeds the differential pair linear region, e.g. outside the linear region may be defined as V_(on)<−0.75 V_(in_max) and V_(in)>0.75 V_(in_max). V_(in_max) max being the target or desired input voltage that generates the target or desired output voltage V_(out_max), required by the subsequent modulator 7. Accordingly, in the illustrated conventional driver circuit, in order to reach the target output voltage (V_(out_max)) V_(in) must equal 1.15*V_(in_max), since V_(in_max) falls outside the linear region of the transfer function of the differential pair Q₃ and Q₄. The linear input voltage range of the differential pair Q₃ and Q₄ may be extended by using degeneration resistor(s) R₁, however, the extended linear range comes at the expense of lower gain. Further extension of the linear range requires increase of the final stage current and degeneration, i.e. higher power consumption.

The embodiment in accordance with the present invention, illustrated in FIG. 4, includes the combination of degeneration resistors R₁ for extended linearity, and an auxiliary circuit, which may be comprised of auxiliary transistors Q_(3b) and Q_(4b), for extending the linear region of the last stage amplifier 4. The last stage amplifier 4 includes a voltage level shifter 11, and a differential pair circuit 12. The voltage level shifter receives the input voltages from the previous VGA 3 _(n) at V_(INP) and V_(INN). Interconnection between the last amplifier stage 4 and the previous VGA stage 3 _(n) requires the voltage level shifter 11 to shift the reference for the input electrical signals. Accordingly, the transistors Q_(1a) and Q_(2a) act as voltage buffers that shift the reference for the electrical signal. A first terminal, e.g. base or gate, of each buffer transistor Q_(1a) and Q_(2a) is connected to the voltage inputs V_(INN) and V_(INP), while the second and third terminals, e.g. collector or drain, and emitter or source, are connected between a voltage source V_(CC) and respective current sources I_(1a) and I_(2a) for biasing the buffer transistors Q_(1a) and Q_(2a). The third terminal, e.g. emitter or source, of each buffer transistor Q_(1a) and Q_(2a) may be connected to a first terminal, e.g. base or gate, of the differential pair Q_(3a) and Q_(4a), respectively, whereby the differential pair Q_(3a) and Q_(4a) transform the input voltages to output currents, which are transformed to output voltages by the load, e.g. the optical modulator 7, at outputs V_(OUTN) and V_(OUTP). The second terminals of the differential pair Q_(3a) and Q_(4a) are connected to the outputs V_(OUTN) and V_(OUTP), and the third terminals of the differential pair Q_(3a) and Q_(4a) are connected to the a respective current source I₃ and I₄ for biasing the differential pair Q_(3a) and Q_(4a). The third terminals are also interconnected by the degeneration resistance R₁.

The differential pair circuit 12 may comprise two additional auxiliary transistors Q_(3b) and Q_(4b) that are biased off, but turn on once the input voltage exceeds a set value, e.g. a maximum input voltage V_(max_linear) of the differential pair Q_(3a) and Q_(4a) that produces a linear output. The auxiliary circuit, e.g. the auxiliary transistors Q_(3b) and Q_(4b), may be used to extend the linear region of the last stage amplifier 4, i.e. beyond that of the differential pair Q_(3a) and Q_(4a). First terminals, e.g. base or gate, of the auxiliary transistors Q_(3b) and Q_(4b) may be connected to the third terminals of auxiliary buffer transistors Q_(1b) and Q_(2b). Third terminals, e.g. source or emitter, of the auxiliary transistors Q_(3b) and Q_(4b) may be connected to ground as degenerated common-emitters, with resistors R₄ and R₅, respectively. The maximum input voltage V_(max_linear) may be determined experimentally for each differential pair or selected based on experience, e.g. a predetermined average, minimum or maximum of a plurality of previous devices.

Ideally, substantially matching the maximum input voltage of the differential pair Q_(3a) and Q_(4a) transfer function, i.e. the maximum input voltage V_(max_linear) that produces a linear output, to the voltage that turns on the auxiliary transistors Q_(3b) and Q_(4b) may extend the linear operation range of the differential pair-based amplifier circuit 12 used in the last stage amplifier 4. Accordingly, as the input voltage approaches the maximum input voltage for the differential pair Q_(3a) and Q_(4a), the auxiliary transistors Q_(3b) and Q_(4b) turn on.

Auxiliary transistor bias input voltage is obtained from the auxiliary voltage buffer circuit, e.g. comprised of auxiliary voltage buffer transistors Q_(1b) and Q_(2b), in parallel with the buffer transistors Q_(1a) and Q_(2a), respectively. First terminals, e.g. gate or base, of the buffer transistors Q_(1a) and Q_(2a) and the respective auxiliary buffer transistors Q_(1b) and Q_(2b) may be connected to the same node, and second and third terminals connected between the voltage source V_(CC) and respective current sources I_(1b) and I_(2b). A DC voltage shift is introduced using shift resistors R₂ and R₃ n series with the respective third terminals of the auxiliary buffer transistors Q_(1b) and Q_(2b). Capacitors C₁ and C₂, which may be in parallel with shift resistors R₂ and R₃, increase the current from the auxiliary voltage buffer transistors Q_(1b) and Q_(2b), at increased frequencies.

For example, when the input signal V_(INP) and V_(INN) is larger than V_(max_linear), e.g. 0.75 V_(in_max or) 0.7 V, the input voltage, buffered by auxiliary voltage buffer transistors Q_(1b) and Q_(2b), respectively, and shifted by shift resistor R₂ and R₃×current source I_(1b) and I_(2b), respectively, raises the voltage of the first terminal, e.g. base, of the respective auxiliary transistors Q_(3b) and Q_(4b), which output a current that is added in parallel with differential pair transistor Q_(3a) and Q_(4a), respectively, thereby extending the linear region of the transfer function of the last stage amplifier 4 and the driver 1.

Process, voltage and temperature variation will change the voltage at which the auxiliary transistors Q_(3b) and Q_(4b) turn on, therefore, the voltage is made controllable by using the voltage drop in the shift resistors R2, R3. For this goal, the bias voltage of the auxiliary transistors Q_(3b) and Q_(4b) at their inputs in sensed by a controller Aux Bias using sensing resistors R7 and R8; this voltage is compared in comparator 11 with a reference voltage V_(REF) while varying variable current sources I_(1b) and I_(2b) (see FIG. 1). Then a feedback loop with controller Aux Bias is used to vary the magnitude of the currents sources I_(1b) and I_(2b), such that the auxiliary transistors Q_(3b) and Q_(4b) turn on at the same desired input voltage magnitude, e.g. V_(max_linear).

The output of the auxiliary transistors Q_(3b) and Q_(4b), e.g. the second terminals, e.g. drain or collector, may be connect to the differential pair output currents, therefore, the total output current is the addition of the differential pair Q_(3a) and Q_(4a) and the auxiliary transistors Q_(3b) and Q_(4b).

The obtained transfer function of the last stage amplifier 4 of FIG. 4, as illustrated in FIG. 5, is more linear, therefore, the target output voltage (V_(out_max)) for the modulator 7 is obtained at an input voltage V_(in) equal to the target input voltage V_(in_max), i.e. with 15% less input voltage.

With reference to FIG. 6, the auxiliary transistors Q_(3b) and Q_(4b) extend the amplifier linear operation. For example, as illustrated by circles in FIG. 6, an output voltage equal to the target output voltage V_(out_max) has 5.2% total harmonic distortion (THD) in a standard implementation, while using the auxiliary transistors Q_(3a) and Q_(4a) reduces the THD to 1.8%. Furthermore, current in the last stage amplifier 4 may increase due to the auxiliary transistors Q_(3a) and Q_(4a) by 2.3%, when the output voltage V_(out) equals the target output voltage V_(out_max), e.g. from 59.17 mA to 61.13 mA. If the standard amplifier is modified to obtain 1.8% THD, its current consumption may increase by 17.7%, e.g. from 59.17 mA to 69.17 mA.

With reference to FIG. 7, any impact on the frequency response due to the use of the auxiliary transistors Q_(3a) and Q_(4a) may be minimized by shielding their capacitance from the output, e.g. using cascode transistors Q_(3a) with Q₅ and Q_(4a) with Q₆. In the illustrated example the second terminal, e.g. drain, of the transistor pair transistors Q_(3a) and Q_(4a), are connected with the third terminals, e.g. source, of the cascode transistors Q₅ and Q₆, while the first terminals, e.g. bases, of the cascode transistors Q₅ and Q₅ are both connected together and to a bias voltage, exemplified by a voltage source V_(case). The example in FIG. 8 uses cascode transistors Q₅ and Q₆, but other circuit techniques are within the scope of the invention.

With reference to FIG. 8, THD varies across frequency; however, the auxiliary transistors Q_(3a) and Q_(4a) reduce overall THD across frequency, making the driver 4 of the present invention an ideal solution for broadband circuits.

The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. 

We claim:
 1. A transmitter driver for preparing input electrical signals for output to a modulator comprising: first and second inputs for inputting first and second input electrical signals; a buffer stage for shifting a reference of the first and second input electrical signals; an amplifier stage receiving the first and second input electrical signals, including: first and second differential pair transistors forming a differential pair connected to the buffer stage; first and second outputs connected to second terminals of the first and second differential pair transistors, respectively, for outputting first and second output electrical signals; first and second auxiliary transistors connected to the second terminals of the first and second differential pair transistors, wherein the first and second auxiliary transistors are capable of turning on when the first and second input electrical signals exceed a maximum input voltage of a linear region of a transfer function of the first and second differential transistors for increasing the first and second output electrical signals, thereby increasing a linear region of a transfer function of the amplifier stage.
 2. The transmitter driver according to claim 1, further comprising a first degeneration resistor extending between third terminals of the first and second differential pair transistors capable of extending a linear voltage range of the amplifier stage.
 3. The transmitter driver according to claim 1, wherein the buffer stage comprises first and second buffer transistors connected to first terminals of the first and second differential pair transistors, respectively.
 4. The transmitter driver according to claim 3, wherein the buffer stage includes third and fourth buffer transistors including first terminals connected to the first terminals of the first and second buffer transistors, respectively, and third terminals connected to the first terminals of the first and second auxiliary transistors, respectively.
 5. The transmitter driver according to claim 4, further comprising first and second shifting resistors connected to the third terminals of the third and fourth buffer transistors, respectively, for shifting voltage applied to the first terminals of the first and second auxiliary transistors, respectively, whereby the first and second auxiliary transistors turn on when the first and second differential pair transistors exceed the maximum input voltage of the linear region of the transfer function of the first and second differential transistors.
 6. The transmitter driver according to claim 5, further comprising first and second capacitors in parallel with the first and second shifting resistors capable of increasing current from the third and fourth buffer transistors at increased frequencies.
 7. The transmitter driver according to claim 1, further comprising second and third degeneration resistors connected to third terminals of the first and second auxiliary transistors, respectively.
 8. The transmitter drive according to claim 1, further comprising a bias voltage connected to the second terminals of the first and second buffer transistors.
 9. The transmitter drive according to claim 1, further comprising a respective current source connected to each of the first and second buffer transistors, and the first and second differential pair transistors.
 10. The transmitter device according to claim 9, wherein the current sources connected to the first and second buffer transistors comprise variable current sources for adjusting the voltage at which the first and second auxiliary transistors turn on.
 11. The transmitter device according to claim 10, further comprising a feedback loop capable of sensing a bias voltage of the first and second buffer transistors, comparing the bias voltage to a reference voltage, and adjusting the current sources connected to the first and second buffer transistors until the first and second buffer transistors turn on at the maximum input voltage of the linear region of the transfer function of the first and second differential pair transistors.
 12. The transmitter device according to claim 1, further comprising first and second cascode transistors connected to the first and second differential pair transistors, respectively.
 13. The transmitter driver according to claim 1, wherein the first terminals comprise a base, the second terminals comprise a collector, and the third terminals comprise an emitter of a bipolar transistor.
 14. The transmitter driver according to claim 1, wherein the first terminals comprise a gate, the second terminals comprise a drain, and the third terminals comprise a source of a MOSFET.
 15. A transmitter comprising: the transmitter driver of claim 1; a laser for generating an optical signal; and a modulator for modulating the optical signal in accordance with the first and second output electrical signals.
 16. The transmitter according to claim 15, wherein the buffer stage of the transmitter driver comprises first and second buffer transistors.
 17. The transmitter according to claim 16, wherein the buffer stage includes third and fourth buffer transistors including first terminals connected to the first terminals of the first and second buffer transistors, respectively, and third terminals connected to the first terminals of the first and second auxiliary transistors, respectively.
 18. The transmitter according to claim 17, further comprising first and second shifting resistors connected to the third terminals of the third and fourth buffer transistors, respectively, for shifting voltage applied to the first terminals of the first and second auxiliary transistors, respectively, whereby the first and second auxiliary transistors turn on when the first and second differential pair transistors exceed the maximum input voltage of the linear region of the transfer function of the first and second differential transistors. 